A murder trial began Tuesday for a man accused of stabbing a Toronto cancer researcher to death in 2015, in an apparently-random attack.

Calvin Nimoh, 24, has pleaded not guilty to first-degree murder in the death of Dr. Mark Ernsting, a 39-year-old biomedical engineer at the Ontario Institute of Cancer Research.

Around 9:30 p.m. on Dec. 15, 2015, while out on an evening walk near his College Park home, Ernsting was stabbed numerous times.

"The Crown is unaware of any connections between the accused and Dr. Ernsting, other than they were both in the same place at the same time," Crown attorney Michael Cantlon said.

Ernsting was stabbed nine times. "Four times in the right back and five times in the head, face and neck,” Cantlon said. “The lethal injury was a stab wound so forceful it broke the blade from the knife handle.”

At the start of the trial Tuesday, Nimoh attempted to plead guilty to manslaughter, but the Crown rejected the plea.

"With his plea of guilty to manslaughter there is no issue to be resolved regarding 'who' stabbed and killed Dr. Ernsting," Cantlon told the jury in his opening address. "However, the issue for you to determine in this case is 'why?'"

Cantlon told the jury Nimoh was with an accomplice to an alleged assault that occured hours before the stabbing. The Crown alleges he robbed and stabbed 65-year-old Glynis Brownsey.

Brownsey told the jury she was visiting Toronto on business and nearly at the home of a friend she was staying with, when she suddenly felt an arm wrap around her chest.

"It was a firm pull toward the body. Then when I turned to see who it was, that's when the arm became very firm and yanked me back against the body," Brownsey testified. "The whole incident was silent. I heard no conversation. No words were spoken. It was just surreal in that sense. I was walking along one minute and the next minute I was being drawn to a body and then I was being punched."

When paramedics arrived to examine her a short while later, she learned she had not been punched, but stabbed in her arm and in her torso. A knuckle on her right hand was broken, she said.

"I was horrified and I thought I was very lucky," she said.

Just before 10:30 p.m. that evening, police arrested Nimoh in the Summerhill area, in connection to Brownsey attack.

"While searching his person, the police located a red handle of a knife and a matching protective sheath. There was no blade attached to the handle. The police detected blood," Cantlon said.

The Crown says the blood matched Ernsting's, and the blade remained in Ernsting's head.

"The evidence called will present two distinct, yet similarly vulnerable members of this community walking after dark," Cantlon said. "Actions speak louder than words. The Crown submits the evidence will lead you to the conclusion that Mr. Nimoh is guilty of first-degree murder."

The Crown alleges that Nimoh carried out the assault on Ernsting on his own.

The trial will continue Wednesday.